Purification of aromatic hydrocarbons



.km. 10, 1939. H. M. SMITH ET AL 2914394335 PURIFICATION OF AROMATICHYDROCARBQNS Filed March 22, 1957 Patented Jan. 10, 1939 v 2,143,036OFFICE PURIFICATION OI AIOHATIO HYDBOCABBONSlhroldhlsinlthl'eterflrandonaandflarry'l.

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Application ms zz, 1m. Serial No. new 2 (mm. (0!. zoo-m .(Granted nnderthe m of March a, 1m, as X amended April 30, 1928: 3'10 0. G. 757) Thisinvention described herein may be. manuiactured and used by or tor theGovernment for governmental purposes, without the payment to us of anyroyalty thereon.

This invention which is a continuation in part of our pendingapplication filed April 26. 1934, Serial No. 722,520, entitledPurification of aromatic hydrocarbons, relates to the refining of crudelight oil and particularLv the light oil produced by the pyrolysis ofgaseous hydrocarbons, as described and claimed in Patent No. 1,995,330,issued March 26, 1935, and entitled Process for treating gaseoushydrocarbons wherein the gases issuing from the reaction tube andconsisting largely of uncracked raw gas unsaturates'such as ethylene andaromatic substances such as benzene, naphthalene, anthracene, pyrene andstill heavier hydrocarbons and possibly small amounts of carbon togetherwith a considerable amount of hydrogen are cooled to condense theheavier hydrocarbons to a liquid or solid condition, passed through anelectrical precipitating chamber to remove the condensates and thenunder suitable conditions of temperature, pressure and rate of flow aredirected to a catalytic hydrogenation chamber from which they are passedto a recovery system. Crude benzene from gas works contain numerousunsaturated compounds which polymerize and resinify upon standing, andit has been found that crude benzene from the pyrolysis of gaseoushydrocarbons turns yellow and deposits gums; indicating that it alsocontains the same or similar unstable compounds as gas-works benzene.

' The usual methods employed for treat ng gasworks benzene result inhigh losses ranging from 5 to 20 percent. Various methods have beensuggested for treatment including the use of: sulfuric, boric andhydrofluoric acids; aluminum,

zinc and iron chlorides; chlorine; sulfur; head and pressure. Oftenseveral of these agencies are combined, but in all cases the principleis to remove the harmful material in some form, and as a consequence,losses result. In some instances the methods are costly or cumbersome.

It is apparent that much of this loss could be prevented if theunsaturated compounds which are responsible for the color and'gums inthe benzene could be converted to useful products. In view of thesefacts it is the object of this invention to provide a method wherebycrude benzene maybe refined without loss, and we have discovered thatthis object may be attained by causing the benzene vapors containing theunsaturated compounds to traverse a nickel catalyst under suitableconditions.

A great many olefins and other unsaturated compounds have beenidentified among the products of gaseous hydrocarbon pyrolysis. Certaintypes of unsaturated compounds, such as the diclefins, are well knowngumforming agents. This property also extends to the cyclo-dioleiins orcyclodienes, and certa n aromatic olefins such as phenyl ethylene orstyrene. Among the compounds which have been found in the liquidsresulting from pyrolysis of gaseous hydrocarbons are cyclohexadiene,methyl cyclohexadiene and styrene. These compounds are unstable and aresubject to polymerization reactions. Richter (Richters OrganicChemistry, Translated by DAlbe, P. Blakestons Son 8: Co. Vol. 11) saysof cyclohexadienes They are easily polymerized and resinified, while ofstyrene he says "it is polymerized to meta-styrene on standing or in thesunlight."

Thus it appears probable that gum formation, at least in crude benzeneproduced by natural gas pyrolysis, is due to the presence of these andrelated compounds. Onetheory dealing with the formation of benzene fromthe pyrolysis of hydrocarbon gases postulates a series of condensationand dehydrogenation reactions, and one of the intermediate productssuggested is a cyclo-' hexadiene. If this theory is correct; and it hasmany facts to support it, then the presence of small amounts of suchcompounds as cyclohexadiene should be expected. The method which we havediscovered for treating crude benzene may be illustrated by reactionswhich 1.3 cyclohexadiene would undergo when subjected to catalytictreatment such as we propose.

1,3 cyclohexadiene, when passed with an inert gas over a nickel catalystat C., is dehydrogenated completely and benzene is produced according tothe following equation:

mo on new no I I HC H Ni HQ C O H H 1,3 cyclohexadiene Benzene HydrogenOn the other hand if the diene is vaporized with a current of hydrogenthen the completely hydrogenated product, cyclohexane, is the result:

i +1; on

. H H: 1,3 cyclohexadiene Hydrogen Cyclohexsne these reactions aretypical for the various unsaturated compounds present in the crudebenzene produced by pyrolysis, and they are the basis for our invention.When hydrocarbon gases are pyrolyzed to produce benzene differentamounts of hydrogen are formed depending upon operating conditions, butthere are also present considerable amounts of methane and possiblyethane. These gases are inert so far as the reactions discussed aboveare concerned, so that the type of reaction which takes place probablydepends upon the hydrogen concentration and operating conditions in thecatalyst chamber. Due to the relatively small amounts of reactivecompounds present and their close relationship to the major productbenzene, we have been unable to determine which type of reaction occurs,but the net overall result from either or both of these reactions is theproduction of gum-free color-stable benzene.

Coming now to a more detailed account of the process, the pyrolyzed gascontaining the lightoil vapors and even naphthalene is desired, but freeof heavier compounds such as anthracene, pyrene and tars, is passed overa nickel catalyst within a temperature range of 180 to 220 0., and at apressure ranging from 0-75 pounds gauge pressure. Here the unsaturatedgumforming compounds are either completely hydrogenated ordehydrogenated according to their individual reactive properties underthe operating condi tions. In either case the gas with the light-oilvapors next enters a light-oil recovery system where the gum-freecolor-stable benzene is recovered. We have been able to recover amountsof treated benzene equivalent to the amount of crude benzene which canbe recovered without treating, so that there is a negligible loss.Untreated benzene rapidly turns yellow, deposits gums and has adisagreeable odor, whereas the treated benzene retains its water-whitecolor indefinitely.

Reference will now be made to Figure 1, which shows a diagrammaticdrawing in elevation of an assembly of apparatus suitable for theconduction of the treating method described. The gas enters the systemat I, passes through the measuring devices 2 into the cracking chamber3. The cracked gas next passes through a high-temperature scrubber orelectrical precipitator l to remove heavy tars and carbon, and then intothe catalyst chamber 5 where the reactions discussed previously occur.The gas and vapors pass from the treating or catalyst chamber to arecovery unit 6 for removing the light-oil (benzene). The light-oil asrecovered here should be suitable for use without any further treatmentexcept distillation to segregate the desired fractions.

In conclusion, it will be apparent, that we have devised a simple,inexpensive method for the purification of crude benzene which affords ayield free from objectional odor, coloring agents and gums.

Having described our invention, what we claim as new and wish to secureby Letters Patent is:

1. A process for converting and otherwise rendering inefiective the gumand color forming constituents present in the tar-free light oilproduced by the catalytic hydrogenation of the pyrolyzed products ofnatural gas after such products have been treated to effect condensingof the heavier hydrocarbons and passed through a high potentialelectrical field to remove the condensates by electrical precipitationand which comprises recycling the products of hydrogenation through theinitial pyrolyzing, condensing and electrical precipitation stages andthen passing the resulting products in a vapor stage and in the presenceof hydrogen over a metallic catalyst at atmospheric pressure and withina temperature range of 180 to 220 0., whereby to produce water white andgum stable benzene.

2. A process for converting and otherwise rendering ineffective the gumand color forming constituents present in the tar-free light oilproduced by the catalytic hydrogenation of the pyrolyzed products ofnatural gas after such products have been treated to effect condensingof the heavier hydrocarbons and passed through a high potentialelectrical field to remove the condensates by electrical precipitationand which comprises recycling the products of hydrogenation through theinitial pyrolyzing, condensing and electrical precipitation stages andthen passing the resulting products in a vapor stage and in the presenceof hydrogen over a metal catalyst at a pressure ranging from 0 to '75pounds gauge pressure and withina temperature range of 180 to 220 C.,whereby to produce water white and gum stable benzene.

HAROLD M. SMITH. PETER GRANDONE. HARRY T. RALL.

